Traditional methods for detecting submerged objects include sonars towed through the water and, in recent years, synthetic aperture radar (SAR). The SAR works by imaging the water surface while a current flows such that wind creates capillary waves over regions of shallow water. Since the water moves at speeds that vary according to depth, and the wind-caused by the capillary waves vary according to water speed, and the radar return from a patch of water is higher for a region occupied by capillary waves than for a region of comparatively smooth water, the SAR can reveal bathymerty. Attempts to harness this technique for discovery of smaller objects, such as sea mines, have yielded disappointing performance. Experimental systems that use lasers to read acoustic signals from the water surface have been demonstrated in recent years. They have shown promise, but suffer from the fact that the laser beam is very narrow, and can be deflected from the receiver by small waves on the water surface. A solution to this signal loss has been elusive, and without it, the system development probably will not proceed.
Sometimes conventional technology incorporates a sonar system used to create wave patterns indicative of the presence of the objects. The current search systems use tethered sonar transmitters which are placed in the water to emit acoustic waves that will be modified when striking underwater objects. These are awkward to deploy and control. This is especially true since the system also includes a laser and detector mounted on an airplane for reading wave patterns that are indicative of underwater objects. In the alternative, magnetic detectors can be used rather than laser detectors.
Unfortunately, both types of conventional detection systems have relatively low search rates, and are often defeated by turbid waters. When using a laser system there are often significant degradation caused by surface waves, as well as other environmental conditions.
The use of synthetic aperture radar (SAR) has been considered for the detection of underwater mines. However, this use of SAR has been considered impractical within the limits of the conventional art, which relies upon the use of DC current passing over the underwater targets, while the SAR is used to find the resulting indications of the underwater object. High speed mine detection and other applications requiring both high speeds and precision have proven inappropriate for conventional uses of SAR, along with traditional liquid agitation techniques.
Much underwater detection, such as bottom-mapping, does not require great resolution, so that conventional systems are still quite suitable. However, there are other applications, such as underwater rescue, mine detection and submarine detection that require a systems capable of rapidly scanning and analyzing indicia signals indicative of underwater objects. Speed is especially critical in combat or emergency situations. Therefore, there is a need for an underwater detection system that avoids the relatively slow operation of conventional systems, while still maintaining high degrees of sensitivity and accuracy.